428 



STRUCTURE OF POLLEN-GRAINS. 



quarters ; generally speaking it approaches the spheroidal, hut it 

 is sometimes elliptical, and sometimes tetrahedral. It varies 

 more, however, when the Pollen is dry, than when it is moist ; for 

 the effect of the imbibition of fluid, which usually takes-place 

 Avhen the Pollen is placed in contact with it, is to soften-down 

 angularities, and to bring the cell nearer to the typical sphere. 

 The Pollen-Cell (save in a few submerged plants) has a thick outer 

 coat surrounding a thin interior wall ; and this often exhibits 

 very curious markings, which seem due to an increased thickening 

 at some points and a thinning-away at others. Sometimes these 

 markings give to the surface-layer so close a resemblance to a 

 stratum of cells (Fig. 230, b, c, d), that only a very careful exami- 

 nation can detect 

 the difference. 

 Fig. 230. The roughening 



of the surface by 

 spines or knobby 

 protuberances, as 

 shown at A, is a 

 very common fea- 

 ife^;i:-'£*M-8i MM m^ShatZli ture ; and this 



seems to answer 

 the purpose of 

 enabling the Pol- 

 len-Grains more 

 readily to hold 

 to the surface 

 whereon they 

 may be cast. 

 Besides these and 

 other inequalities 

 of the surface, 

 most Pollen- 



Grains have what 

 appear to be pores 

 or slits in the 

 outer coat, vary- 

 ing in number in 

 different species, 

 through which 

 the inner coat protrudes itself as a tube when the bulk of its contents 

 has been increased by imbibition ; it seems probable, however, that 

 the outer coat is not absolutely deficient at these points, but is 

 only thinned-away. Sometimes the pores are covered by little 

 disk-like pieces, or lids which fall-off when the Pollen-Tube is pro- 

 truded. This action takes-place naturally when the Pollen-Grains 

 fall upon the surface of the Stigma, which is moistened with a 

 viscid secretion ; and the Pollen-Tubes, at first mere protrusions of 





Pollen-grains of, — a, Altkcea rosea ; b, Cobcea 

 scandens; c, Passijlora ccerulea ; d, Ipomcea 

 purpurea. 



